If every generation since your great great grandfather had 3 children, then you can expect your great great grandfather to have 81 descendants in your generatio

If every generation since your great great grandfather had 3 children, then you can expect your great great grandfather to have 81 descendants in your generation (most of whom would have a different surname - to a first approximation you only retain the surname if your connection to him is on the father's side at every level). I'm guessing you wouldn't know all of them, and in the olden days people had more kids so it could easily be a lot higher than that. Isn't it possible that the people running the brewery are also great great grandchildren of your great great grandfather?